The enhancement of liquid–liquid extraction with high phase ratio by microfluidic-based hollow droplet

In this study, a one-step microfluidic capillary device for the enhancement of mass transfer process with high phase ratio by hollow droplet was developed. Herein, a fluorescent material rhodamine B was used as the transportation substance, which gets transferred from an aqueous phase to an oil phase. Qualitative demonstrations of the extraction process are presented. During the droplet formation stage, the fluorescent brightness of the droplets becomes stronger for a droplet with and without a bubble in it. During the droplet moving stage, for both flow conditions, the fluorescence intensity increases along the outlet channel. At the latter half of the outlet channel, the addition of gas greatly enhances the mass transfer process. The mean overall volumetric mass transfer coefficients kLa increases with increasing gas flow rate; this is mostly because of the sharply increased specific area. Moreover, the kLa value of gas–liquid–liquid hollow droplet flow increases around 10–60 times compared to that of a liquid–liquid droplet flow system. The length of extraction equipment needed to reach 95% extraction efficiency is reduced to around 10–1000 times when gas microbubbles are introduced. Based on the experimental data, a theoretical model has been built up for the potential prediction of the enhancement of the extraction by adding gas microbubbles. The effective diffusion coefficient is introduced to combine the convective mass transfer factor into this model. The modelling results fit well with the experimental data. All the abovementioned results present a practical method for the enhancement of the extraction process with high phase ratio systems, which has potential applications in analytical chemistry, micro-extraction, and biological extraction.